1. Field of the Invention
The present invention relates to a digital broadcast receiver and, more particularly, to a method and an apparatus for reducing channel switching delay in a digital broadcast receiver.
2. Description of the Related Art
A digital broadcast receiver is an apparatus that restores original digital data from a broadcast signal transmitted according to a digital transmission scheme. A digital broadcast transmitter, i.e., a broadcast station area (hereinafter, referred to as a head end) converts an analog signal into a digital signal comprised of 0 and 1 using digital technology, compresses the converted signal together with other information, and then transmits the compressed signal according to a digital transmission scheme. Then, the digital broadcast receiver receives and converts the transmitted signal into an original video and audio.
Compared to analog technology, digital technology is usually robust to noise, needs less transmission power, allows use of error correction, and has less degradation in transmission, copy, and accumulation. In addition, digital technology enables high band compression on a video/audio signal and facilitates search, processing, and editing of information. Digital broadcast using such digital technology is advantageous in that it is robust to noise and realizes efficient information transmission compared to conventional analog TV.
Due to recent developments in digital broadcast technology, photos and voice taken/recorded in a studio can be transmitted almost losslessly to a user's home together with a variety of additional information, which is not possible in analog broadcasting. Thus, a user can enjoy broadcast content of interest in various ways. For example, while watching a sports game on TV, a user can obtain information regarding the players in real time.
In digital broadcasting, a highly efficient data compression method, such as a Moving Picture Experts Group (MPEG) compression method, is used, and thus, unlike in analog broadcasting, it takes a considerable amount of time for a screen to become stable after switching broadcast channels. This channel switching delay is inconvenient to users.
That is, in analog broadcasting, a user can watch a broadcast program within about 0.2 seconds after switching channels. However, in digital broadcasting, it takes about ten times more time than in analog broadcasting to normally provide a broadcast program to a user after switching channels, which the user may perceive to be very inconvenient.
In the case of switching channels in a conventional digital broadcast receiver, a tuner waits for a predetermined amount of time after a phase locked loop (PLL) circuit is stabilized and then performs a forward error correction (FEC) operation on a digital signal to correct an error that may occur when a demodulation unit demodulates the digital signal. The forward-error-corrected digital signal is input to an MPEG decoder and then decompressed, thereby retrieving an analog image signal. Even after the forward-error-corrected digital signal is decompressed, the tuner also waits until the analog image signal becomes stabilized and then outputs the stabilized analog image signal.
In general, no specific detection methods are used to detect the amount of time required to stabilize a signal obtained as a result of performing the above operations in the conventional digital broadcast receiver. In the receiver, it usually takes 2-3 seconds for the signal to become stable enough to provide a broadcast program after switching channels. In particular, an MPEG encoder compresses video and audio signals as a combination of intra coded frames (I frames), bidirectionally predictive coded frames (B frames), and predictive coded frames (P frames). The numbers of B frames and P frames inserted between a pair of adjacent I frames vary according to a compression rate. The higher the compression rate, the greater the numbers of B frames and P frames inserted between a pair of adjacent I frames. For example, ‘IBBBPBBBP . . . ’ corresponds to a higher compression rate than ‘IBBPBBP . . . ’
Here, an I frame is coded data that is decoded independently of other frames, a P frame is coded data that is decoded With reference to an I frame or P frame temporally previous thereto, and a B frame is coded data that is decoded with reference to an I frame or P frame temporally previous thereto or subsequent thereto.
An I frame is used as a reference frame when retrieving an image signal and is generated at intervals of about 0.5-1.5 sec. Conventionally, an MPEG decoder is generally set to have a delay time of more than 1.5 sec to wait for an I frame, which results in an increased amount of time required to switch channels. In order to reduce such channel switching delay, the MPEG decoder may be set to have a delay time of, for example, 0.5 sec. In this case, however, an abnormal screen may be temporarily provided to a user after switching channels, which is highly inconvenient and undesirable.